1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, printer, or facsimile apparatus, and a control method thereof.
2. Description of the Related Art
In general, an electrophotographic image forming apparatus forms an image by transferring and superimposing developing material images (toner images) of respective colors formed on different image carriers (photosensitive drums) onto the surface of an intermediate transfer member (transfer belt) or that of a print material held on a conveyance belt. When transferring and superimposing the developing material images, alignment (registration) is important. In the image forming apparatus, however, the photosensitive drums and transfer belt suffer rotational nonuniformity depending on the device precision or the like. The positions of toner images of respective colors shift in the rotational direction, resulting in color misregistration in the formed image. This problem becomes serious especially in a tandem type color image forming apparatus.
Such position errors (registration errors) are classified as a DC color registration error and AC color registration error. The DC color registration error is a registration error in which the positions of the leading and trailing ends of a formed image in the moving direction do not match owing to rotation of the transfer belt or the like. The AC color registration error is a registration error in which the magnitude or direction of a position error periodically fluctuates mainly due to a rotating member such as the photosensitive drum or belt driving roller. Of these registration errors, the DC color registration error can be suppressed by detecting registration patches of respective colors by a registration sensor, and correcting the leading end position of a formed image or correcting the magnification in the conveyance direction.
For the AC color registration error, there is known an image forming apparatus which detects rotational fluctuations of the rotating member using an encoder attached to the rotating shaft of the photosensitive drum or the like, and feeding forward or back the detection result to the driving motor to reduce the rotational fluctuations. Even by this control, however, the speed of surface of the photosensitive drum fluctuates owing to an eccentric component that is generated from misalignment of the central axes of the photosensitive drum and photosensitive drum driving shaft. Also, the speed of surface of the transfer belt fluctuates due to the eccentricity caused by misalignment of the rotating shaft of the driving roller or the like, and thickness nonuniformity. For the AC color registration error arising from composite factors, the following methods are being examined.
For example, Japanese Patent Laid-Open No. 62-59977 proposes a method of reducing a registration error amount by locking the phases of registration errors caused by the eccentricity of the photosensitive drums of respective image forming units. In this method, however, when the amplitude of the AC component of rotational fluctuations caused by the eccentricity of the photosensitive drum is different between the photosensitive drums, the AC component still remains.
To solve this problem, for example, Japanese Patent Laid-Open No. 2000-250284 proposes a method of suppressing the AC component in one cycle of the photosensitive drum. Japanese Patent Laid-Open No. 10-333398 proposes a method of suppressing the AC component in one cycle of the photosensitive drum, a plurality of cycles until the photosensitive drum is driven, and one cycle of the transfer belt. Japanese Patent No. 3186610 proposes a method of suppressing the AC component by setting the exposure position and transfer position to be opposite to each other by 180° on the photosensitive drum. Japanese Patent Laid-Open No. 2004-317538 proposes a method of forming an electrostatic latent image free from any position error on the photosensitive drum. In this method, the position error amount is estimated from the result of detecting fluctuations of the speed of surface of the photosensitive drum, and the position error is corrected from a displacement amount smaller than the minimum unit of the resolution to an arbitrary displacement amount.
However, these conventional techniques have the following problems. For example, in the methods disclosed in Japanese Patent Laid-Open Nos. 2000-250284 and 10-333398 and Japanese Patent No. 3186610, when the rotational speed of the photosensitive drum is corrected to suppress the AC component of rotational fluctuations, the difference in the speeds of surface between the transfer belt and the photosensitive drum at the transfer position fluctuates with time. Owing to the speed difference, the state of a stick-slip between the transfer belt and the photosensitive drum changes, making it difficult to effectively suppress the AC component. In the method disclosed in Japanese Patent Laid-Open No. 2004-317538, even if electrostatic latent images are formed at constant intervals on the photosensitive drum at the exposure position, a position error is generated in an image transferred onto the transfer belt under the influence of the stick-slip at the transfer position. Therefore, the conventional techniques cannot satisfactorily suppress the AC color registration error generated under the influence of an unpredictable random stick-slip.